Nowadays, there is a growing concern about depletion and rising cost of oil resources, and studies are conducted on transition to new energy resources from oil resources being one of the reasons for the global warming. One of new energy sources is solar thermal power generation in which sunlight is collected and used as energy.
As the sunlight collecting system, a beam-down sunlight collecting system is known in which sunlight is reflected by heliostats being reflection mirrors toward above a center portion of the system and the reflected light is converged to a receiver (heat receiving portion) installed at a lower portion of the system by a large reflection mirror called a center reflector (see Patent Document 1, for example).
The receiver may include pipes and the like in which a heating medium (molten salt or the like, for example) circulates. In this case, the heating medium having received the thermal energy from sunlight at the receiver is configured to be sent to a power generation unit. The power generation unit includes, for example, a power generator which generates steam by utilizing heat of the heating medium and supplies the steam to a steam turbine to generate electricity, and other devices.
As a method for improving power generation efficiency with use of the sunlight collecting system, an invention in which a heat collecting receiver is formed in a pot shape (in the shape of a cavity) is disclosed (see Patent Document 2). The invention described in Patent Document 2 is configured such that sunlight having entered the pot is reflected multiple times to transfer heat to the heating medium efficiently and that the sunlight is held inside the pot. This configuration significantly increases the efficiency of photovoltaic power generation.
However, the pot-shaped receiver described in cited document 2 has a structure in which a single pipe is shaped into a pot shape, and thus faces problems that manufacturing the receiver is very difficult and the manufacturing costs are high. In addition, the receiver is large in size, and thus transporting the receiver of an integrated type is not easy and constructing the receiver on site is a demanding work.
Further, a failure of the receiver such as a pipe breakage requires the entire receiver to be replaced. When a beam-down sunlight collecting system of 400 m to 500 m square is constructed for example, the diameter of the pot-shaped receiver is about 8 m to 10 m, which is significantly large.
Moreover, sunlight falls on the inside of the receiver unevenly, which causes uneven temperature distribution thereof and thus leads to uneven thermal expansion of the pipe forming the receiver. Hence, there is a problem that the gap appears in the receiver or the pipe expands in a small space resulting in a failure such as a crack in the pipe in some cases.
Further, in the case of a failure of the receiver such as a pipe breakage, the entire receiver needs to be replaced because the receiver is formed of a single pipe, which leads to a large replacement work. In addition, there is a problem of a high cost of preparing the new receiver.
Further, in the sunlight collecting system, molten salt such as sodium nitrate which turns into a liquid phase at 150° C. to 500° C. is most commonly used for the heating medium. When the molten salt is used, the molten salt needs to be withdrawn from the receiver and moved to a thermal insulating tank on a cloudy day and during nighttime. This is because the molten salt, when cooled, is solidified to cause clogging in the receiver tube. When the molten salt is recovered, it takes time to withdraw the molten salt from the receiver formed of a helical pipe described in cited document 2. Particularly, there is a problem that the technique is incapable of dealing with an emergency where the molten salt should be recovered promptly.